Tracking is one of the functions that must be provided in an optical pickup head. Currently, many optical heads use an electro-mechanical voice coil actuator to physically translate the optical head, and the voice coil is part of a servo loop used to correct tracking error. The tracking actuator generates a corrective cross-track motion in response to the tracking error signal generated by the optical head itself. Unfortunately, the mass and size of the electro-mechanical actuator increases the cost of the optical head, and, due to inertia, limits the bandwidth of the optical head. Furthermore, coupling of the mechanical motions of the tracking and focus servos can cause unwanted crosstalk between these two channels. Accordingly, it will be appreciated that it would be highly desirable to have a low mass tracking actuator in a waveguide optical pickup head. It is also desirable to have a deflector or nonmoving part as an electrically controlled fine tracking actuator in a waveguide optical pickup head.
Waveguide optical pickup heads are disclosed by Kataoka et al in U.S. Pat. No. 4,802,153 and by Arimot et al, Applied Optics, Vol. 29, 1990, pp. 247-250. Both describe integrated optic heads which employ surface acoustic wave (SAW) deflectors. Unfortunately, the SAW waveguide deflector requires a high power, variable frequency RF generator, and the RF noise generated may interfere with other parts of the system. The SAW provides low diffraction efficiency deflection, and diffraction efficiency is dependent on the diffraction angle.
Another manner of deflecting light in waveguides is disclosed by Kaminow and Stulz IEEE JQE, Aug. 1975, pp. 633-635, wherein tilted surface electrodes are deposited on a planar electro-optic waveguide to effect an electrically tunable deflection of the beam. Unfortunately, the planar electro-optic prism deflector relies on the rather weak electro optic coefficient of inorganic materials such as LiNbO.sub.3, and employs inefficient interaction between surface electrode fringing fields and guided light. Consequently, high voltages are required to drive this device. Also, the planar electro-optic prism deflector has an aperture limited to 100 .mu.m or so compared to a required aperture of several millimeters for a waveguide optical pickup head.
Planar electro-optic waveguide beam deflectors have also been described by Himel et al in IEEE Photonics Technology Letters, Vol. 3, 1991, pp. 921-923, and by Martin in U.S. Pat. No. 3,923,376. Both describe planar waveguide devices in which the optical beam is deflected in a direction normal to the plane of the waveguide. In the former case, an electrically conductive substrate supports a multimode optical waveguide of electro optic material which varies in electrical resistivity as a function of its thickness. Application of an electrical potential between an upper planar electrode and the substrate causes a gradient in the refractive index normal to the plane of the waveguide due to the gradient in electrical resistivity of the electro-optic material. This gradient in index of refraction causes the out-coupled light to deflect in a direction perpendicular to the plane of the waveguide. In the latter case, an electro-optic material is placed atop a waveguide such that guided light in the waveguide leaks into the high index electro-optic material Application of an electrical potential between planar electrodes which are placed atop the electro-optic material and on the bottom of the substrate which supports the waveguide causes the light which leaks into the electro-optic material to deflect in a direction normal to the waveguide plane. The primary disadvantage of these devices is that the electrodes are placed on either side of the substrate requiring high voltage to effect significant deflection. Another disadvantage of these devices is that the wavefront quality of the out-coupled beam can be distorted by the existence of multiple modes in the former disclosure and by the use of leaky modes in the latter. Finally, the fact that the beam is deflected in a direction perpendicular to the plane of the waveguide makes it more difficult to construct truly compact waveguide pickup heads.
Electro-optic deflection in liquid-crystal waveguides has been disclosed by Sheridan and Giallorenzi, Journal of Applied Physics, Vol. 45, Dec. 1974, pp. 5160-5163, and by Hu et al in IEEE Journal of Quantum Electronics, Vol. QE-10, Feb. 1974. Although these devices exhibit continuous deflection through relatively large angles (greater than 20 degrees), light guided in liquid-crystal waveguides suffers severe propagation losses due to scatter caused by fluctuations in the liquid-crystal director orientation. Furthermore, the bandwidths of these devices are limited by electrically induced hydrodynamic instabilities.
Recently, strides have been made in the area of nonlinear optical organic (NLO) materials. For example, U.S. patent application Ser. No. 735,550, filed Jul. 25, 1991 by Penner et al., for Improved Conversion Efficiency Second Harmonic Generator, discloses means of forming poled noncentro-symmetric organic molecules by means of the Langmuir-Blodgett (L-B) technique. The use of electrically poled nonconcentro-symmetric organic molecules in guest host polymer structures has been advanced U.S. Pat. No. 4,900,127 discloses another means of obtaining nonlinear optic organic polymers D.J. Williams disclosed specific nonlinear organic molecules with large second order hyperpolarizabilities in an article "Organic Polymer and Non Polymeric Materials with Large Optical Nonlinearities", Anqew. Chem., Int. Ed. Engl. Vol. 23 1984, pp. 690-703. Other references to organic nonlinear optical media in the form of transparent thin films are described in U.S. Pat. Nos. 4,694,066; 4,536,450; 4,605,869; 4,607,095; 4,615,962; and 4,624,872.
It is desirable to have a low mass tracking actuator that is a deflector or nonmoving part useful as an electrically controlled fine tracking actuator in a waveguide optical pickup head. It is also desirable to utilize nonlinear optic organic materials and reduce the need for a high power, variable frequency RF generator and the associated RF noise.